Merge branch 'rr/am-quit-empty-then-abort-fix'

Recent "rebase --autostash" update made it impossible to recover
with "git am --abort" from a repository where "git am" without mbox
was run by mistake and then was killed with "^C".

* rr/am-quit-empty-then-abort-fix:
  t/am: use test_path_is_missing() where appropriate
  am: handle stray $dotest directory